1. Field of the Invention
The present invention relates to a sheet feeding apparatus for feeding a sheet, an image reading apparatus, and image forming apparatus such as a copying machine, a laser-beam printer, and a facsimile machine.
2. Description of the Related Art
An example of the conventional sheet feeding apparatus used for the image forming apparatus such as the copying machine will be described with reference to FIG. 10. FIG. 10 shows a frictional pad separation type sheet feeding apparatus. The sheet feeding apparatus includes a feeding roller 151, a lifter 152, and a separation pad 153. The feeding roller 151 is rotatable in a direction of an arrow a. The lifter 152 is elevatable in directions of arrows b and c while supporting a front end side of the sheet. The separation pad 153 is pressed against a feeding surface 151a, which is of an outer peripheral surface of the feeding roller 151, to form the nip portion n.
The separation pad 153 is made of a rubber material such as urethane rubber and EPDM (ethylene, propylene, diene, monomer) in a plate shape. The separation pad 153 is bonded on an upper surface of a pad mount 153b rotatable about a shaft 153a and on a position where the pad mount 153b faces the feeding roller 151. The pad mount 153b is biased by a coil spring 153c, which presses the separation pad 153 against the feeding roller 151.
Before a sheet-shaped original S is fed, a front end portion of a bundle of plural originals S stacked on an original tray 154 is inserted between the lifter 152 and the feeding surface 151a of the feeding roller 151. The lifter 152 is elevated in the direction of the arrow b of FIG. 10 to sandwich the inserted front end portion of the bundle of originals S by the lifter 152 and the feeding roller 151.
When the feeding roller 151 is rotated in the direction of the arrow a of FIG. 10, an uppermost original S1 on the original tray 154 is fed in a direction of an arrow d by frictional force acting between the feeding roller 151 and the original S. When the original S passes through the nip portion n between the feeding roller 151 and the separation pad 153, the front end of the bundle of originals S is individually separated to prevent double-sheet feeding by the frictional force of the separation pad 153, and only one sheet is separated and delivered.
After the separation and feeding, the front end of the original S1 reaches a conveyance roller pair (not shown) and sandwiched by the conveyance roller pair arranged on the downstream side of the feeding roller 151 in an original feeding direction. The lifter 152 is located at the pressing position with respect to the feeding roller 151 while conveyance force is imparted, which prevents the conveyance roller pair from conveying the original S1. Therefore, the lifter 152 is previously rotated in the direction of the arrow c to lower to a retracted position.
In the sheet feeding apparatus having the above configuration, the good separation and feeding are achieved by utilizing a delicate difference in frictional force among the feeding roller 151, the separation pad 153, the original S, and the like.
However, as shown in FIG. 10, hardness of the separation pad 153 is relatively increased in the separation pad 153 having the configuration in which the rubber material is formed in a plate shape and bonded onto the upper surface of the pad mount 153b in a planar manner. Therefore, a region of the nip portion n formed between the feeding roller 151 and the separation pad 153 is narrow in the original feeding direction. In other words, the nip portion n has a small area. Accordingly, a friction coefficient μ of the nip portion n is remarkably decreased from the viewpoint of durability, and there is a fear that the double-sheet feeding is generated because the front end of the bundle of originals S is not sufficiently separated. On the other hand, in techniques described in Japanese Patent Application Laid-Open (JP-A) No. 9-124174 and JP-A No. 10-316265, the wide nip portion whose region in the original conveyance direction is broadened is formed in order to improve the double-sheet feeding preventing performance. However, in the techniques described in JP-A No. 9-124174 and JP-A No. 10-316265, since the nip portion is formed in the wide nip portion, vibration generated in the wide nip portion is increased while the double-sheet feeding preventing performance is improved by the wide nip portion. Therefore, the conveyance performance is decreased due to the vibration itself, and there is a fear that judder of the nip portion is generated due to the vibration.